Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation (thermal inkjet (TIJ)). In another process, known as continuous inkjet (CIJ), a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught, and returned to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
It is well known in the art that printed images generated using inkjet technology may not be very stable with regard to environmental effects of light and ozone. Another problem with such printed images is that they may be easily scratched during handling and may not be resistant to typical aqueous solutions such as coffee, fruit punch, etc. Scratches on the printed images are particularly objectionable to consumers as these defects detract from the overall image quality.
Several methods have been used to protect printed images generated using inkjet technology. One method is referred to as lamination and involves the application of a clear durable film over the printed image. Another method involves printing onto specially designed inkjet recording elements that are subsequently fused to form a clear durable film over the printing image. Both of these methods have drawbacks in that an integral or peripheral fusing station is required.
Another method for increasing the durability of printed images involves application of an overcoat composition onto the surface of the printed image using a brush, roller, sponge, etc. As the composition dries, a clear durable film is formed. This method is useful in a variety of commercial printing applications but is considered too impractical and undesirable for consumer use in the home.
To this end, the use of a colorless ink supplied in a printhead of an inkjet printer has become increasingly popular. The printhead containing the colorless ink is typically part of the same carriage assembly containing colored inks, and the printer is instructed to jet the colorless ink either simultaneously with or, after the colored inks are jetted. U.S. Pat. Nos. 6,428,157 and 6,206,586 describe an inkjet printing apparatus for applying a composition capable of forming a continuous protective overcoat film.
The colorless inks, also known as overcoat solutions or clear ink compositions, are typically formulated with polymer, water, and other components commonly used in aqueous-based inkjet ink formulations, for example, humectants, organic solvents, surfactants and biocides. U.S. Pat. Nos. 6,723,784; 6,604,819; and 6,543,888 describe a coating liquid and image recording method that provides a transparent topcoat for recordings. Jetting an aqueous suspension of fine polymer particles onto a recorded image forms the topcoat.
U.S. Pat. No. 6,561,644 describes an inkjet printing method for enhancing the durability of an image by applying a water-dispersible polyester resin over the printed image. U.S. Pat. No. 5,640,187 describes an inkjet recording method having a step in which a solution containing a resin having an acid value of 30 to 100 and soluble in alkali is discharged to a recording member prior to or after the ink is discharged.
U.S. Pat. No. 6,894,107 describes inkjet ink compositions containing fusible polymers that may be used in a colorant containing ink, or as a separate clear ink composition, which are thermally fused after printing of the colored inks. GB Patent Publication No. 2,337,482 and EP Patent Publication No. 1,186,439 describe natural and synthetic resin compositions that are used to overcoat inkjet recorded images using an inkjet printer. US Patent Publication No. 2004/0202838 describes a durability enhancing overcoat composition comprising addition polymers having an acid number from between 110 and 300. International Publication No. WO2006/052707 describes a durability enhancing overcoat composition comprising a polyurethane resin. US Patent Publication No. 2002/0156153 describes the use of polymeric additives in fixatives for inkjet printing in order to improve print quality and image permanence attributes on plain paper.
Another approach to improving the durability of a printed image is to incorporate synthetic or natural resins into the colored inks prior to ejection from the inkjet printer. Approaches that utilize high molecular weight polymeric dispersants and latexes in the pigmented ink compositions have been employed in drop-on-demand piezoelectric printhead printers directed at photographic quality pictorial image reproduction (U.S. Pat. Nos. 6,713,531; 6,180,691; and 6,866,707).
To date, these approaches have not afforded printed images with excellent durability and scratch resistance.